Ligand-receptor assays or immunoassays are well-known in the art. Since their introduction in 1971, such assays have been utilized in a variety of applications to detect minute amounts of hormones, drugs, antibodies, and other substances suspected of being present in a given fluid sample. In this regard, researchers equipped with enzymes, antibodies, gene probes, and other reagents have made it possible to create chemical detection schemes for almost any compound of interest in a great diversity of applications. Among these applications are: commercial production of pharmaceuticals and food stuffs; food safety; diagnosis and treatment of disease in medical, veterinary, and agricultural environs; and detection and eradication of toxins in the environment. Common to all such applications is the requirement that chemical detection be performed in a timely, reliable, and cost effective manner.
Generally, bioassay schemes are developed and commercialized in formats suitable for use in laboratories equipped with general purpose instrumentation. Examples of these formats include immunoassay and DNA hybridization performed in test tubes, cuvettes, microtiter plates, columns, and electrophoretic gels. These formats usually include elaborate operational procedures and require frequent calibration using several calibrants which contain the analyte of interest at different concentrations. As a consequence, the high cost and complexity of operation associated with such formats limits widespread utilization thereof.
To address such drawbacks, developers and end users of immunoassays are increasingly replacing conventional bioassay formats which use test tubes, cuvettes, microtiter plates, columns, and electrophoretic gels with thin film chromatographic devices known as test strips. As is known in the art, the majority of test strips used for immunochemical detection of compounds are so called lateral flow test strips in which sample and reagents flow within the plane of the test strip. Advantageously, assays configured in a test strip format can produce rapid results, are simpler to operate, and are more cost-effective than conventional formats. Additionally, such test strip assays may be utilized by unskilled laborers and can produce results on-site (i.e., outside a laboratory facility).
Generally, such assays rely on the binding of analytes by receptors to determine the concentration of such analytes in a given sample and are typically characterized as either competitive or non-competitive. Non-competitive assays generally utilize receptors in substantial excess over the concentration of analytes to be determined in the assay. Typical of such non-competitive immunoassays include sandwich assays, which detect the presence of an analyte by binding two receptors thereto. In such arrangement, the first receptor, which is typically an antibody is bound to a solid phase such that when the analyte is present, such analyte becomes affixed thereto. A second receptor having a label covalently attached thereto, which may comprise a radioactive, fluorescent, enzymatic, dye or other detectable moiety (collectively referred to as tracers), is introduced to the assay which consequently binds to the bound ligand, to the extent the ligand is present, and thereafter produces a signal consistent with the presence of such ligand. If the sample does not contain the molecules of interest, the labeled receptor is carried past the immobilized receptor without reacting which, as a consequence, will not cause a change in the membrane. Such non-competitive immunoassays are primarily useful for the detection of large molecules such as proteins, large hormones or molecules which have multiple binding sites, such as human chorionic gonadotropin (HCG) and typically will not work with small molecules that have only one binding site.
Competitive assays, in contrast, generally involve competition between a ligand present in a given sample, and a ligand analog having a tracer/label covalently linked thereto to permit detection for a limited number of binding sites provided by the ligand receptor, which typically comprises an antibody bound to a solid phase. Such assays are particularly suited to detect smaller molecules, such as drugs and drug metabolites. In this context, drug analogs are utilized that have been covalently bound to a protein which is then immobilized on a membrane. Antibody specific to the drug is then labeled and immobilized on a porous pad. When a sample is added which is suspected of containing a given analyte, such sample dissolves the labeled antibody and carries it into contact with the immobilized drug-protein region. If there is little or no drug in the sample, a large amount of the labeled antibody is bound to the immobilized drug-protein region which, consequently, produces a detectable signal. If the sample contains a high amount of drug, little or no labeled antibody is bound to the immobilized drug-protein region and thus in turn gives little or no signal.
Today, rapid immunoassays generally consists of an adhesive-covered plastic backing onto which several porous pads and a piece of protein-binding membrane are attached. The membrane typically contains a section that has been impregnated with a binding partner (i.e., a receptor or ligand analog). A second pad is typically provided which contains a labeled target molecule or labeled antibody protein-binding membrane. When a sample suspected of containing a target ligand is contacted with the immunoassay, such sample dissolves the labeled element or tracer and the capillary action of the protein-binding membrane subsequently draws the sample with tracer dissolved therein into contact with the impregnated binding partner. When this reaction occurs, there is a change in the appearance of the binding membrane, with the difference providing a qualitative indication of the presence or absence of the ligand suspected of being present in such sample.
Typical examples of this form of test strip are those which visually display two parallel lines (known as capture lines) on a test membrane. Capture lines consist of immobilized capture reagents or receptors which are preapplied to the test membrane during its manufacture. In this regard, both virtually all prior art assays, whether competitive or non-competitive, typically deploy a receptor immobilized on a membrane, as assessed above. A schematic representation of the construction of a typical lateral flow test strip is as follows: EQU reagent pad//test membrane/capture line/test membrane/capture line/test membrane//absorbent pad
where:
symbol / designates a phase boundary within a single chromatographic medium; and PA1 symbol // designates a union of two separate mediums (chromatographic or other medium).
One of the two capture lines serves as an indication that the test strip performance has not been compromised. In this regard, such capture line serves an important function by providing quality assurance and integrity of the assay, which is generally considered necessary insofar as individual test strip performance can vary greatly. The second of such capture lines becomes visible only when the sample contains an amount of analyte in excess of a minimum concentration (threshold concentration). Exemplary of such prior art systems and methodologies include the immunoassay systems and test strips disclosed in U.S. Pat. No. 5,658,723, issued on Aug. 19, 1997, to Oberhardt entitled "Immunoassay System Using Forced Convection Currents" and U.S. Pat. No. 5,712,170, issued on Jan. 27, 1998, to Kouvonen, et al. entitled "Test Strip, Its Production and Use", the teachings of each of which are expressly incorporated herein by reference.
Unfortunately, despite their cost-effectiveness and simplicity of use, typical test strip format assays are less accurate, less precise, and less sensitive to analyte presence than conventional formats. As a result of such drawbacks, the application of test strip format assays has been limited to semi-quantitative or qualitative assays. Among the more significant factors that contribute to the inaccuracy and imprecision of test strip format assays include the manufacture and use of capture lines. As is widely recognized, the manufacture of consistently uniform capture lines requires elaborate material control and manufacturing processes with rigid specifications that must operate within narrow tolerances. Moreover, to function properly, most test strip formats require that the analytes to be detected must be uniformly captured in a precise geometry at a precise location on the test strip and that factors such as the ambient humidity present at the time of test strip manufacture, type of membrane utilized in such manufacturing process, and a capture reagent-receptor itself contributing greatly to assay inaccuracies and false readings. A detailed discussion regarding the drawbacks associated with the binding of protein capture reagents in immunochromatographic assays can be found in Jones, Kevin D., "Troubleshooting Protein Binding in Nitrocellulose Membranes", Part I, IVD Technology, Volume V, No. II, March-April 1999, pages 32-41 and Part II, IVD Technology, Volume V, No. III, May-June 1999, pages 26-35, the teachings of which are expressly incorporated herein by reference.
It is therefore desirable to devise an alternative lateral flow device which can capture analyte at a precise location and in a precise geometry without the use of preapplied capture lines. There is further a need for an assay that has greater sensitivity in reproduceability than prior art assays and methods and is likewise inexpensive, relatively easy to manufacture, and capable of being utilized for a wide variety of applications. There is still further a need in the art for such an assay that can identify the presence of two or more suspect ligands in a given sample. It is also desirable to devise an analytical instrument which can provide a quantitative analysis of a captured analyte on a lateral flow assay device.